The present invention relates to a solid polymer electrolyte fuel cell, and more particularly to improvements in sealing structure between a gasket disposed on the periphery of a membrane electrode assembly and conductive separator plates.
The most typical example of solid polymer electrolyte fuel cells comprises: an electrolyte membrane-electrode assembly (MEA) composed of a polymer electrolyte membrane, a gasket which is formed of a sealing material and supports the periphery of the electrolyte membrane, an anode attached to one face of the electrolyte membrane, and a cathode attached to the other face of the electrolyte membrane; an anode-side conductive separator plate and a cathode-side conductive separator plate sandwiching the MEA; and gas supply means for supplying a fuel gas and an oxidant gas to the anode and the cathode, respectively. The important problem with this kind of fuel cells is cross leakage of the gasses which takes place in the vicinity of gas manifold apertures. In the vicinity of oxidant gas manifold apertures, cross leakage of the gasses occurs because the gasket sags into a fuel gas flow channel of the conductive separator plate. The sagging consequently creates two leak paths leading to the oxidant gas manifold aperture from the anode. One of the leak paths is created by separation of the gasket from the anode side of the separator plate, and the other is created by separation of the gasket from the electrolyte membrane as the result of the sagging of the gasket. Likewise, in the vicinity of fuel gas manifold apertures, gas leakage occurs because the gasket sags into an oxidant gas flow channel of the conductive separator plate.
In order to solve this problem, the inventors of the present invention made the following proposal in WO 02/061869. The entire disclosure thereof including specification, claims, drawings and summary is incorporated herein by reference in its entirety. That is, the disclosure is a method in which a plurality of through holes 2 are arranged in the periphery of an electrolyte membrane 1 as illustrated in FIG. 1, and a gasket is integrally joined to the periphery of the electrolyte membrane by injection molding so as to include the through holes. In this method, the portion of the gasket covering one face of the electrolyte membrane is connected to the portion covering the other face thereof at a portion covering the edge of the electrolyte membrane and at the through holes, so that it is possible to eliminate the cross leakage of the gasses caused by separation of the gasket from the electrolyte membrane. Also in this method, the gasket is provided with ribs formed between the anode and the oxidant gas manifold apertures, and the ribs are fitted into grooves formed in the corresponding positions of a separator plate to prevent the separation of the gasket from the anode side of the separator plate. Likewise, ribs formed on the gasket are fitted into grooves of a separator plate to prevent the separation of the gasket from the cathode side of the separator plate. These ribs of the gasket not only mate the gasket to the separator plates but also function as flowing paths of molten resin in molding. Thus, the ribs are indispensable for gaskets that are thin and injection molded.
However, the prevention of the gas leakage by fitting the ribs of the gasket into the grooves of the separator plates has been found insufficient.
In injection molding, molded articles are inevitably subject to mold shrinkage. Since the degree of mold shrinkage varies depending on the molding materials and the shapes of the molded articles, it is normally difficult to predict beforehand. Thus, in case the degree of mold shrinkage has been beyond the prediction, there arises a problem of the ribs of the gasket not fitting into the grooves of the separator plates properly. Therefore, in the above-described structure of fitting the ribs of the gasket into the grooves of the separator plates, the gasket needs to be molded beforehand, and then the separator plates need to be designed based on the actual measurement of the mold shrinkage of the molded gasket. Since the separator plates are composed mainly of metal or carbon, even the molded separator plates are hardly subject to mold shrinkage. Thus, designing the separator plates so as to mate with the molded gasket is a rational process. This process, however, has a disadvantage that the design of the separator plates must be done each time the rate of mold shrinkage is changed, for example, by the change of the gasket material.
In the above-described structure, the sealing between the gasket and the separator plates is basically surface to surface sealing except for the mating portions, and both the gasket and the separator plates therefore need to have sufficient surface accuracy. However, on the surface of an injection molded article, gate marks and ejector pin marks are left inevitably. The heights of the marks are usually approximately a few tens of microns depending on the mold structures and materials. In the above-described structure of the fuel cell, when the gate marks and ejector pin marks are left on the rib portions or the standard thickness portion (the portion without the ribs) of the gasket, except for the case where the gasket is extremely elastic, clearances are produced between the separator plates and the gasket to cause cross leakage or outward leakage of the gases. This problem is common particularly in the case of using molded separator plates. Since the separator plates have almost no elasticity, such surface irregularities need to be compensated solely by the gasket. That is, it is necessary to use a highly elastic material for the gasket. However, such a highly elastic material has a problem in that it usually has poor mechanical strength and therefore tends to creep.
Further, in the above-described surface to surface sealing, a sufficient surface load needs to be applied onto both the gasket and the separator plates. Hence, another problem arises in that the clamping pressure of the cell stack must be heightened unnecessarily. This also involves a problem of requiring unnecessarily large-scale clamping members such as end plates, bolts, springs, etc., the large-scale clamping members giving a negative effect in terms of the volume.
In one aspect of the present invention, small ribs for sealing are formed on the gasket in order to ensure the sealing between the gasket and the separator plates instead of sealing the gasket and the separator plates in a surface to surface manner.
In another aspect of the present invention, while ribs and other moldings of the conventional gasket are retained for ensuring moldability and mechanical strength, small ribs for sealing are formed on portions of the gasket which would conventionally come in contact with the separator plates in a surface to surface manner. The mechanical strength required for the gasket is bending strength, tortional strength or the like, and particularly a strength which allows the gasket not to sag into the gas flow channels of the separator plates. The former ribs are hereinafter referred to as dummy ribs since they make no direct contribution to the sealing, and the latter ribs are hereinafter referred to as seal ribs.
The present invention is directed to a polymer electrolyte fuel cell comprising a unit cell, the unit cell comprising: an electrolyte membrane-electrode assembly (hereinafter referred to as MEA) comprising a polymer electrolyte membrane, a gasket covering the periphery of the electrolyte membrane, an anode attached to one face of the electrolyte membrane, and a cathode attached to the other face of the electrolyte membrane; and an anode-side conductive separator plate and a cathode-side conductive separator plate sandwiching the MEA therebetween.
The gasket and the anode-side and cathode-side conductive separator plates have a pair of fuel gas manifold apertures, a pair of oxidant gas manifold apertures and a pair of cooling water manifold apertures. The gasket comprises a dummy rib which at least partially surrounds one of the seal ribs which will be described later on the anode-situated side and a dummy rib which at least partially surrounds one of the seal ribs on the cathode-situated side. The separator plates comprise a groove into which the dummy rib is fitted loosely such that there is a clearance therebetween. The groove into which the dummy rib is fitted loosely specifically refers to a groove which is larger than the dummy rib both in width and depth/height and into which the dummy rib is fitted without being bound.
The gasket comprises, on the anode-situated side, a seal rib which surrounds a fuel gas flow section extending from one of the fuel gas manifold apertures through the anode into the other of the fuel gas manifold apertures and seal ribs which surround each of the cooling water manifold apertures. The gasket preferably comprises seal ribs surrounding each of the oxidant gas manifold apertures on the anode-situated side.
The gasket further comprises, on the cathode-situated side, seal ribs which surround each of the fuel gas manifold apertures and the cooling water manifold apertures. The gasket preferably comprises a seal rib which surrounds an oxidant gas flow section extending from one of the oxidant gas manifold apertures through the cathode into the other of the oxidant gas manifold apertures.
The above-described seal ribs are pressed against the separator plates by clamping pressure of the cell stack to form gas sealing sections.
The anode-side conductive separator plate comprises, on the anode-facing side, a fuel gas flow path which communicates with the pair of fuel gas manifold apertures, and the cathode-side conductive separator plate comprises, on the cathode-facing side, an oxidant gas flow path which communicates with the pair of oxidant gas manifold apertures. The fuel gas flow path and the oxidant gas flow path communicate with the fuel gas flow section and the oxidant gas flow section of the gasket, respectively.
The dummy rib and the seal ribs are formed on both sides of the standard thickness portion of the gasket. The expression xe2x80x9cstandard thicknessxe2x80x9d as used herein refers to the thickness of the gasket which does not include the height of the rib. As described above, the dummy rib has the function of improving the moldability of thin gaskets. Thus, it is preferable that the dummy rib on the anode-situated side substantially surround the seal rib surrounding the fuel gas flow section, and that the dummy rib on the cathode-situated side substantially surround the seal rib surrounding the oxidant gas flow section. More preferably, the gasket further comprises, on the anode-situated side, dummy ribs surrounding each of the seal ribs surrounding the oxidant gas manifold apertures and the cooling water manifold apertures, and further comprises, on the cathode-situated side, dummy ribs surrounding each of the seal ribs surrounding the fuel gas manifold apertures and the cooling water manifold apertures. In molding the gasket, the gate point is preferably formed so as to be connected to the dummy rib. Specifically, in the case of pin gate, pin-point gates are desirably formed on the dummy ribs. In the case of side gate, gates are formed so as to be connected to the dummy ribs, so that molten resin injected from the gates flows along the dummy ribs first, and then flows so as to form the standard thickness portion, seal ribs and other moldings. The width and height of the dummy ribs are primarily determined by the fluidity of the molding resin. With regard to the shape of the dummy ribs, fine adjustments may be made in view of the mechanical strength that will be described later, but in consideration of the thickness of the separator plate, the appropriate height of the dummy ribs is approximately 0.3 to 0.8 mm from the standard thickness portion. As to the thickness of the separator plate, it is preferable to design the separator plate so as to have sufficient mechanical strength even when it is grooved on both sides for receiving the dummy ribs.
It will be apparent to those skilled in the art to which the present invention pertains that the dummy ribs are not necessarily required for gaskets of which standard thickness portions have such a thickness as to secure moldability and mechanical strength.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.